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Name: Stephen
Status: educator
Grade: 4-5
Location: MA
Country: USA
Date: Fall 2011


Question:
My class of fifth graders and I had a spinning, levitating magnet toy, and we tested it with a balance beam. First we compared it to tiny rocks, with the magnet NOT levitating, to balance the scale. Next we turned the floating magnet around so that it was levitating above its base. We noticed then that the scale changed, and the magnet side was higher, indicating that it had a higher mass than before (even though the mass of the object did not actually change or transform between when we turned the floating magnet over. Can you help me explain how this measurement changed?

Replies:
OK, Stephen and company,

I'm finding it a little hard to be sure I understand your picture right.

I have a levitating spinning magnet toy too. I presume it is about the same kind of thing you have. It consists of a big heavy magnetic block which sits on the table, over which a small magnetic top spins and hovers (if you spin and lift it just right using your hands).

What part of this two-piece toy did you put on the scale? There is a strong magnetic force between those two pieces. . . . . . . . . If you put only the top on the scale, then the magnetic force from the block would add or subtract some force from the weight. This would change the measured weight by a large amount, and the amount would depend on where the block was, how far away, and on the orientation of the top. So if you were weighing the top alone and the big block was nearby, about 6 inches away, the orientation-dependent changes you told me about could happen.

For the top to weigh the same in both orientations, the big magnet block has to be really far away, more than 1 foot away. Better to be more than 3 feet away to be sure the force cannot reach the top you are weighing. . . . . . . . . If you put both the big block and the top on the scale together, then I think they would weigh the same whether the top was point-up and hovering, or point-down and clinging to the block.

If you really saw a change depending on orientation while measuring both parts together, then I think there had to be some magnet or magnetic metal near the scale that you did not notice. Try getting some big nails and see how close they have to be to tilt the balanced scale. Or try some bigger piece of iron which sticks to magnets. Sometimes a watch on somebody's wrist interferes with the weighing. . . . . . . . . I do not understand how you weighed a magnet floating above its magnet-block. (unless you weighed both together). By itself, the floating top will not push on the scale unless it is touching the scale. What the floating top pushes down on is the big block, and the block then pushes down a little harder on the scale by an amount equal to the weight of the top. That is true and same-amount whether the top is floating above or clinging to the top surface of the block. Weighing both together should work right. Weighing each separately should work too, but only if they are quite far apart and the top is resting on the scale not floating. However you do it, always watch out for sneaky magnetic stuff nearby. It can be tricky to weigh a magnet.

I hope that helps. This can be a little confusing to talk about. Sometimes things are clearer with pictures, sketches, diagrams, which these letters cannot do.

Jim Swenson


There are two possible sources of this error that come immediately to mind.

1) The scale is made of a material that is attracted by the magnet. 2) When you turn the magnet over it is now pulling down on the toy and not up.

Hope this helps. R. W. "Mr. A." Avakian


Stephen

Sorry, but I am having difficulty visualizing your experiment.

I have seen toys where one magnet hovers over another because their like magnetic poles (North to North; or, South to South) were pointing toward each other and I do not know why anything has to spin. Are you talking about a magnet or a gyroscope?

In any case, the magnet should weigh the same regardless of whether it is levitating, spinning, or not.

Perhaps you should check your weight scale.

Sincere regards, Mike Stewart


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